Hollow cathode gas discharge device

ABSTRACT

The hollow cathode gas discharge device is configured with a maximized cathode-to-anode area ratio to operate in a lowpressure glow discharge mode to generate a plasme of adequate density from which electrons or ions can be extracted and accelerated. This permits the gas pressure to be kept low to avoid Paschen breakdown in the high voltage acceleration region. The invention herein described was made in the course of or under a Contract or subcontract thereunder with the Department of the Navy.

Elite ttes Patet [191 Kneclitli 11] 3,31,52 Aug. 2%, 1974 HOLLOW CATHODEGAS DISCHGE DEVECE [75] Inventor: Ronald C. Knechtli, Woodland Hills,

Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: May 25, 1973 [21] Appl. No.: 363,904

315/111 [51] Int. Cl. 1101] 7/24 [58] Field of Search 3l3/DIG. 8, 231,74, 183, 313/191, 209, 210, 207, 187; 315/111 [56] References CitedUNITED STATES PATENTS 3,262,003 7/1966 Allen et a1. 313/187 3,411,03511/1968 Necker et al. 313/187 r 85 g Vo OTHER PUBLICATIONS Low PressureGlow Discharge by G. W. McClure, Applied Physic Letters, Vol. 2, No. 12,June 15, 1963.

Primary Examiner-Harold A. Dixon Attorney, Agent, or Firm-W. H.MacAllister; Allen A. Dicke, Jr.

[57] S CT The hollow cathode gas discharge device is configured with amaximized cathode-to-anode area ratio to operate in a low-pressure glowdischarge mode to generate a plasme of adequate density from whichelectrons or ions can be extracted and accelerated. This permits the gaspressure to be kept low to avoid Paschen breakdown in the high voltageacceleration region.

The invention herein described was made in the course of or under aContract or subcontract thereunder with the Department of the Navy.

9 Claims, 3 Drawing Figures Vg L gs PAIENIE AUBZO m4 SEW? l M 2 Fig. 5.

IGNITOR HOLLOW CATHODE GAS DISCHARGE DEVICE BACKGROUND This invention isdirected to a hollow cathode gas discharge device having a low-pressureglow discharge plasma from which can be extracted electrons or ions forhigh voltage acceleration.

High energy electron and ion beams are used in a variety of equipment,such as irradiation equipment, TEA gas lasers, and ion thrustors.Various electron and ion sources are available. Glow discharge plasmacontain both ions and electrons, are widely used for various purposes,and can be used for this purpose. However, when they are employed inequipment where accelerating voltages are required, differential pumpingis usually needed to keep the gas pressure in the accelerating regionlow enough to prevent Paschen breakdowns in the beam under highaccelerating voltages.

Among the prior art, attention is called to the thin wire anode, hollowcathode discharge described by G. W. McClure, AMERICAN PHYSICS LETTERS,Volume 2, No. 12, page 233, June 15, 1963.

SUMMARY In order to aid in the understanding of this invention, it canbe stated in essentially summary form that it is directed to a hollowcathode gas discharge device which has a minimized internal gas pressureresulting from a maximized cathode-to-anode area ratio consistent withmaintenance of discharge so that maximized accelerating voltages can beapplied to an extracted beam without Paschen breakdwon and without needfor differential pumping.

It is thus an object of this invention to provide a gas discharge devicewhich provides a glow discharge gaseous plasma as a source for eitherions or electrons. It is another object to provide a gas dischargedevice wherein plasma discharge occurs at a minimized pressure. It isanother object to provide a hollow cathode gas discharge device withmaximized cathode-to-anode area ratio so that a satisfactory glowdischarge can be maintained at minimum gas pressure. It is a furtherobject to provide an auxiliary ignition anode within a hollow cathodegas discharge device which operates at minimized pressure so that theglow discharge can be initiated.

Other objects and advantages of this invention will become apparent froma study of the following portion of the specification, the claims, andthe attached drawmgs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of ahollow cathode gas discharge device, in accordance with this invention,as attached to a laser for ionization of laser gas by means of the highenergy electron beam from the said hollow cathode gas discharge device.

FIG. 2 is an enlarged section, with parts broken away, taken generallyalong the line 22 of FIG. 1.

FIG. 3 is a transverse section through another embodiment of the hollowcathode gas discharge device of this invention showing the gas dischargedevice as an ion source.

DESCRIPTION In FIG. 1, the hollow cathode gas discharge device is a highenergy electron source 10. It is shown as being attached to a lasercavity 12 for the ionization of the gas thereof. The laser cavity isshown as being in association with laser mirrors 14 and 16. In FIG. 2,the laser cavity 12 contains sustainer discharge electrode 18, which ispart of a conventional laser. The laser cavity 12 is the cavity of a gaslaser, of which the TEA laser in U.S. Pat. No. 3,702,973 is an example.Furthermore, U.S. Pat. No. 3,577,096 discloses a gas laser, and U.S.Pat. No. 3,641,454 discloses an electron beam-ionzed gas laser toillustrate the fact that a gas laser can be electron beam-ionized. Seealso the article by A. J. Beaulieu in APPLIED PHYSICS LETTERS, Vol. 16,page 504, 1970. The entire disclosures of these patents and this articleare incorporated herein in their entirety.

Laser cavity housing 12 has secured to the side thereof electron sourcehousing 20. Housing 20 serves as a shell around the remainder of theelectron source structure to serve as a vacuum envelope therefor. Oneside of housing 20 is wall 22, which is a common wall with laser cavityhousing 12. Wall 22 has a thin foil section 24 which serves as anelectron transmission window. Metals are suitable for these housings.The foil window section 24 is as thin as possible to permit electronpassage with maximum freedom, but also to maintain the vacuum integrityof housing 20. Foil section 24 can be mechanically supported to aid inits support of the pressure differential between the interiors of thetwo housings. The pressure within electron source 10 is made independentof the ambient by means of housing 20, which is closed all the wayaround. It can be provided with a vacuum pump and/or gas supply means tomaintain the interior of housing 20 at the appropriate pressure and withthe proper gas atmosphere. The maintenance of as low a pressure withinhousing 20 as is consistent with the maintenance of a plasma dischargeis a feature of this invention and is discussed in more detail below.The geometry of the various parts is related to the maintenance of thedesirably low pressure. Hollow cathode 26 is mounted within housing 20on suitable electrically insulative structural supports. Cathode 26carries webs 28 and 30 on which are mounted insulators 31 and 33. On theinside surface of the insulator, toward the interior of cathode 26, ismounted perforated anode 32. On the other side of the insulators ismounted perforated control grid 34. The thin foil window section 24 isin line with and faces the perforated electrodes. Window section 24 isspaced from control grid 34 and is adapted to be connected as anelectron accelerator electrode.

Cathode webs 36 and 38 extend inward over the exposed portion of theinsulators 32 and 34 and extend over the mounting edges of theperforated anode electrode 32 so that only the perforated section isvisible from the interior of cathode 26. The effective cathode surface40 has an area which extends around the interior of cathode 26 out tothe facing edges of cathode webs 36 and 38. The opening 42 between thefacing edges of cathode webs 36 and 38 is the effective anode area. Webs44 and 46 protect the outer surface of insulators 32 and 34 by carryingthe cathode potential around the outer, protected surfaces of theseinsulators. The presence and length of cathode webs 36 and 38 isoptional. They should extend at least over the exposed insulator ifinsulator protection is wanted, but they may cause beam focusing.Similarly, webs 44 and 46 need extend only to the outer insulator faces,and may cause beam focusing if they extend as far as they are shown inFIG. 2.

The structure of electron source also includes an ignition electrode 48.Ignition electrode 48 is preferably in the form of a thin wire. Itextends substantially through the center of the cathode space. When thecathode 26 is in the form of an elongated tube, as shown, the ignitionelectrode conveniently extends along the length of the structure.

Power sources are connected to provide the necessary currents foroperation. Power source 50 is connected between cathode 26 andperforated anode electrode 32 to maintain the anode electrode positivewith respect to cathode surface 40 to maintain the plasma of the glowdischarge within the interior of the cathode. Voltage is in the order of300 to 600 volts, and current is between about 10* to 1 amp per squarecentimeter of effective cathode area for the type of discharge desired.Ignition power supply 52 is connected between the cathode and ignitionelectrode 48. When ignition is desired, ignition power supply 52provides a positive pulse on ignition electrode 48. A pulse in the orderof 500 to 1,000 volts and in the order of l microsecond time duration isconvenient. Control grid power supply 54 is connected between anodeelectrode 32 and control grid 34 to bias the control grid with respectto the anode electrode. The control grid can be made negative up to avoltage exceeding the discharge voltage of power supply 50 to cut offelectron flow. When not employed as a turn-off device, the control gridpower supply is usually operated so that control grid 34 is at apotential close to that of the anode electrode 32. It can be eitherpositive or negative.

Accelerator power supply 56 is connected between anode electrode 32 andfoil window section 24. The window section is made positive toaccelerate the electrons. In accordance with this invention,accelerating voltages of in excess of 150 kilovolts can be achieved.

The hollow cathode electron gun of FIG. 2 is able to generate a plasmaof adequate density, up to about 10 electrons and ions per cubiccentimeter, from which electrons can conveniently be extracted andaccelerated without causing Paschen breakdown in the high voltageacceleration region between control grid 34 and foil window section 24.To avoid such breakdown, the gas pressure of the discharge has to bekept relatively low. The pressure is typically below about 50 microns ofmercury pressure column for helium and lower for other gases. Theconfiguration of electron source 10 permits operation at such a low gaspressure, because most of the discharge volume is enclosed by the hollowcathode surface, because the anode area is kept much smaller than thecathode and because the anode area is essentially flush with the cathodearea. The enclosure of the discharge volume by the hollow cathodesurface leads to optimum utilization of the ions generated in theplasma, which substantially all fall back onto the cathode where theygenerate the secondary electrons needed to sustain the discharge. Thesecond and third features, the small anode-to-cathode area ratio and theessentially flush configuration of the anode surface with respect to thecathode surface, are essential to permit sustaining the discharge downto low pressures and are further discussed below.

Presuming that the plasma discharge is established, the volume insidethe hollow cathode, defined by the effective cathode surface area 40 andextending across between webs 36 ad 38, is filled with plasma which haspotential close to that of the most positive electrode, the perforatedanode electrode 32. This results from the fact that the electronmobility is much larger than the ion mobility. The discharge voltagetherefor ap pears mostly across the cathode sheath which exists betweenthe cathode surface and the plasma. The cathode sheath thickness is muchsmaller than the diameter of the cathode. This is typical of a coldcathode glow discharge. To sustain the discharge in steady state, therate of ion generation has to equal the rate of ion loss. This conditiondetermines both the lowest pressure at which the discharge can besustained and the discharge voltage.

For the plasma densities found in cathode 26 when operating in thedesired mode, typically up to about 10 ions per cubic centimeter, ionloss is due predominantly to the ion flux to the cathode. There isnegligible ion loss due to volume recombination. The ions reaching thecathode are accelerated through the cathode sheath to an energycorresponding to the discharge voltage. As noted above, this istypically several hundred volts for the desired cold cathode glow modedischarge. Upon impact on the cathode, these ions produce secondaryelectrons. The secondary electrons, in turn, are accelerated through thecathode sheath to the full discharge voltage. In the low pressureregime, the electron mean-free path will be much longer than thedistance between opposite cathode surfaces. Most of the acceleratedelectrons will, therefore, traverse the discharge volume, be reflectedon the opposite cathode surface, and oscillate back and forth betweenopposite cathode surfaces in the hollow cathode volume until theyeventually make an inelastic collision. Such inelastic collisions have ahigh probability of being ionizing collisions. The probability for anelectron to reach the anode before having made such an ionizingcollision increases with decreasing gas pressure, but decreases withdecreasing anode area for a given cathode area. As a result, a smallanode area is important to minimize lowest pressure at which thedischarge can be sustained in this mode. Furthermore, it can beappreciated that this process can take place effectively only if theoscillating electrons are not intercepted by an anode surface. This iswhy a flush anode surface with respect to the cathode surface isimportant, while an anode surface intruding into the hollow cathoderegion would be detrimental.

In a particular structure, the effective cathode surface area was 250square centimeters, and the opening which corresponds to the effectiveanode area was 30 square centimeter. The cathode material was stainlesssteel. The gas in the chamber was helium, and a wellcontrolled dischargecould be sustained down to a helium gas pressure of less than 20milli-Torr. This pressure is suitably low for a plasma cathode gasdischarge device with high voltage electron or ion acceleration.

While the discharge configuration described above and illustrated inFIG. 2 is suitable for sustaining the discharge at the desired low gaspressure, it is inadequate to permit reliable ignition at this lowpressure. This is due to the fact that the vacuum electric fieldexisting between the cathode and an anode which is about flush with thecathode is quite unfavorable. Any initial electron present inside thehollow cathode will be rapidly focused toward the anode and will becollected before it has a chance to make an ionizing collision. Thus,the avalanche necessary to start the discharge cannot take place. Thisis a different situation than in the presence of the plasma because, inthe presence of the plasma, the vacuum field does not exist. With theplasma discharge taking place, most of the electric field exists only inthe cathode sheath region, and the electrons are not focused toward theanode. Thus, while the flush anode configuration is desirable for anumber of applications, the ignition electrode 48 is provided forpractical ignition.

The success of this ignition can be understood by realizing that, whenthe wire diameter is made thin enough, typically less than 1 millimeterin diameter, the probability for an electron which is accelerated towardthe ignition anode electrode 48 being collected depends upon the initialazimuthal electron velocity. Under practical conditions, having thesmall diameter wire, such an initial electron under the influence of thevacuum field will be accelerated toward the ignition wire, but will havea high probability to miss it. Under these circumstances, it becomestrapped in an orbit around this wire until it makes an ionizingcollision and initiates the avalanche required to ignite the hollowcathode discharge. Once the discharge is ignited, it can be readilytransferred from the auxiliary ignition anode electrode 48 to perforatedanode electrode 32. This can be simply accomplished by keeping theperforated anode electrode 32 at or above the discharge voltage andletting the ignition wire anode electrode 48 voltage fall below thedischarge voltage once ignition has taken place.

It will be understood that, even with the thin wire anode ignitionelectrode 48, ignition is predicated upon the appearance of an initialelectron to start an avalanche. In the least favorable case, thegeneration of this initial electrode will depend upon cosmic rayionization. At a gas pressure below 50 microns, and with gas volumeshaving dimensions in the order of centimeters, the rate of generation ofsuch electrons can be as low as on the order of 1 per second. This canresult in a statistical time delay for ignition of the same order. Thisstatistical time delay can be readily reduced to the order ofmicroseconds or less by artificially producing the needed initialelectrons. One means of doing this is to incorporate a low-intensityradioactive source in the discharge region.

Electrons are extracted from the plasma by means of the relativepositive polarity of anode electrode 32. Electrons passing through theperforations of the anode electrode 32, which acts as an extractiongrid, are first accelerated in substantially space-charge limited flowin the extraction and control region between electrodes 32 and 34. Theyare further accelerated, once past control grid 34, by the high voltageaccelerating field applied between window 24 and grid 34. It is seenthat electrodes 32 and 34 are at approximately the same potential. Witha distance between electrodes 24 and 34 in the order of 2.5 centimeters,and with helium as the gas in the space at a pressure of 50 milli-Torror less, accelerating voltage in excess of 150 kilovolts has beenapplied without either Paschen or vacuum breakdown. The maximumaccelerating voltage which can be applied between window 24 and grid 34is determined by the conditions for both Paschen breakdown and vacuumbreakdown. The vacuum breakdown voltage is essentially determined by thedistance d between the high voltage electrodes 24 and 34. For a voltageon the order of kilovolts, a practical minimum value for d is on theorder of 2.5 centimeters. The Paschen breakdown voltage is determined bythe value of the product p'd of the gas pressure p and the electrodespacing d. In the low pressure region of interest for the hollow cathodedischarge electron (or ion) guns, the Paschen breakdown voltageincreases with decreasing value of the product pd. For helium and thedevices described herein, the Paschen breakdown voltage will exceed l50kilovolts for values of p-d typically smaller than 0.4 Torr-centimeter.Now it is observed that increasing the vacuum breakdown requires anincrease in the electrode spacing d, while maintaining the Paschenbreakdown voltage at a selected value requires keeping the 1d productconstant; hence, the need to increase d results in a need to decreasethe gas pressure p. This is why the ability of the hollow cathodedischarge described above to operate at low pressures is especiallyvaluable for this application. A typical practical set of values is anacceleration voltage up to about 200 kilovolts for a maximum helium gaspressure of about 50 microns of mercury pressure column and an electrodespacing of about 4 centimeters. For helium gas pressure of about 30microns, an extracted current density up to several amperes per squarecentimeter has been obtained with a stainless steel cathode, with adischarge voltage in the order of 300 to 500 volts, and an extractiongrid current smaller than or of the same order of magnitude as theextracted electron current.

It is noted that the cross-sectional configuration of cathode 26 neednot be cylindrical. A rectangular configuration is also satisfactory.The only key conditions to be satisfied are that theeffective anode areais much smaller than the effective hollow cathode area, that the cathodesubstantially enclose the plasma space, and that the anode besubstantially flush with the cathode surface.

One application of the plasma cathode electron gun 10 is as an electronsource for a gas laser with high energy electron ionization. In modernlasers of this type, large area high voltage electron guns are required.As compared to the thermionic cathode electron guns presently used forthis application, the plasma cathode electron gun described above hasthe following advantages. Small leaks in the thin metal window can betolerated, provided that the enclosure pressure is maintained at 10 to10' Torr range. Thermionic cathodes require at least two orders ofmagnitude lower pressure. Furthermore, accidental loss'of vacuum wouldhave no serious consequences with plasma cathodes; it is usuallycatastrophic with thermionic cathodes. Finally, the plasma cathode isnot sensitive to electronegative impurity gases, in contrast tothermionic cathodes. The plasma cathode does not require heat-up time.The discharge in the plasma gun can be started within microseconds priorto the initiation of a high energy beam. The plasma cathode electrodegun structure can be maintained at much lower temperature than that ofthermionic cathodes. No inherently delicate heater elements arerequired. The plasma cathode electron gun can readily be scaled to largesize without major difficulties; i.e., no unwieldy heater power, nodifficulties with structural rigidity, etc. The eventual cost of theplasma cathode gun is expected to be lower than that of a comparablelarge area thermionic cathode, due to its inherent structural simplicityand potentially greater reliability. The plasma cathode gun does notrequire power-consuming heater elements. The power required for thedischarge in the plasma gun constitutes only a small fraction of thehigh energy electron beam power. In pulsed duty, the average powerconsumption can be lower than that of an equivalent thermionic cathode.

Another useful application of the plasma cathode electron gun describedabove is for industrial highenergy electron irradiation equipment.Electron irradiation is sometimes applied to polymer compositionmaterials to cause polymerization and can be used for other chemicaluses.

Referring to FIG. 3, ion source 60 is shown therein. Ion source 60 has ahousing 62 which maintains a vacuum in the vacuum space 64 therein.Cathode 66 is mounted in and is insulated with respect to housing 64.Cathode 66 is the same as cathode 26. However, cathode 66 has anextraction grid 68 at the cathode potential. Pressure is maintainedwithin the cathode space '70 at an appropriate value so that a plasmadischarge can be initiated by ignition electrode 72 and maintained bydischarge-sustaining anode 74, the latter being essentially flush withthe cathode surface. For the same reasons as previously described, thepressure is maintained as low as practical within cathode space 70consistent with the maintenance of a low pressure plasma discharge incathode space 70. As ions drift from the plasma to extraction grid 68,they are accelerated by ion-accelerating grid 76 toward target 78.

Ignition pulse power supply 80 is connected between cathode 66 andignition electrode 72 to produce a pulse which initiates the discharge.Discharge power supply 82 is connected between cathode 68 anddischarge-sustaining anode 74 to maintain the previously described lowpressure plasma glow discharge. Acceleration power supply 84 isconnected between cathode 66 and accelerator grid 76. Target 78 is atabout the same potential as accelerator grid 76, or is more negative,and thus is connected to the negative side of accelerator power supply84 or to the negative side of a separate accelerating power supply 85.

Ion source 60 thus produces the low pressure ion and electron-generatingdischarge which was previously described, and ions can be extracted andaccelerated from the plasma. The low pressure in space 64 again permitshigher accelerating fields without Paschen breakdown. The extractiongrid, and a control grid, if desired, can be best made according to theknown techniques developed for electron bombardment and thrustors andion sources. The key difference between the ion source 60 and the priorion sources is the low pressure ion-generating discharge which issustained by means of the hollow cathode structure and operatingconditions described above, without the need for a magnetic field or ahot cathode.

One advantage in using a flush anode configuration to sustain thedischarge rather than a thin wire such as is used for ignition is theeasier cooling of the flush anode resulting in its ability to sustain ahigher average discharge current. A higher average discharge currentproduces a higher plasma density and permits extraction of a higheraverage ion current. Another advantage of the configuration object ofthis invention over a thin wire anode is the fact that it results in amore uniform plasma density distribution, leading to a more uniformcurrent density distribution for the extracted ion beam.

This invention having been described in its preferred embodiment, it isclear that it is susceptible to numerous modifications and embodimentswithin the ability of those skilled in the art and without the exerciseof the inventive faculty. Accordingly, the scope of this invention isdefined by the scope of the following claims.

What is claimed is: l. A hollow cathode plasma discharge devicecomprising:

walls defining a hollow cathode space, said walls comprising an anodewall and a cathode wall, said anode wall and said cathode wall togetherdefining the exterior boundaries of a low-pressure glow dischargeplasma, said anode wall being positioned so that it does notsubstantially intrude into the plasma, the ratio of anode area tocathode area being smaller than unity for low-pressure glow dischargeplasma mode operation;

means for initiating plasma discharge interiorly of said space so thatplasma discharge can be ignited by said initiation means and thentransferred to said anode wall;

one of said walls being perforated so that particles can be extractedthrough said perforation into space exterior of said perforated wall;

an accelerator electrode positioned exteriorly of said hollow cathodespace to accelerate particles passing out through said wall perforation;and

a vessel enclosing at least said perforation and said acceleratorelectrode to maintain pressure within said vessel and within saidcathode at subatmospheric pressure to cause conditions between saidperforation and said accelerator electrode to be outside the breakdownregion of the Paschen curve for the particular gas.

2. A hollow cathode plasma discharge device comprising:

walls defining a hollow cathode space, said walls comprising an anodewall and a cathode wall, said anode wall and said cathode wall togetherdefining the exterior boundaries of a low-pressure glow dischargeplasma, said anode wall being positioned so that it does notsubstantially intrude into the plasma, the ratio of anode area tocathode area being smaller than unity for low-pressure glow dischargeplasma mode operation;

an auxiliary anode positioned interiorly of said space for initialignition of the plasma discharge so that plasma discharge can besustained by said anode wall;

one of said walls being perforated so that particles can be extractedthrough said perforation into space exterior of said perforated wall; anaccelerator electrode positioned exteriorly of said hollow cathode spaceto accelerate particles passing out through said wall perforation; and

a vessel enclosing at least said perforation and said acceleratorelectrode to maintain pressure within said vessel and within saidcathode at subatmospheric pressure to cause conditions between saidperforation and said accelerator electrode to be outside the breakdownregion of the Paschen curve for the particular gas.

3. The device of claim 2 wherein an additional perforated electrode orconducting mesh is placed between said perforated anode and saidaccelerator electrode to serve as a control grid controlling the currentof extracted electrons, the potential of said control grid being betweenthe cathode potential and the potential of the accelerator electrode.

4. The device of claim 3 wherein said perforated wall is a perforatedanode wall so that electrons are the particles extracted from theplasma.

5. The device of claim 3 wherein said vessel includes an electrontransmissive window, and said electron transmissive window is saidaccelerator electrode so that electrons accelerated to said window aretransmitted through said window into an atmosphere of arbitrary pressureand composition.

6. The device of claim 4 wherein said vessel includes an electrontransmissive window, and said electron transmissive window is saidaccelerator electrode so that electrons accelerated to said window aretransmitted through said window into an atmosphere of arbitrary pressureand composition.

7. The device of claim 3 wherein said perforated wall is a perforatedcathode wall so that ions can be extracted from the plasma, saidaccelerator electrode being spaced from said perforated cathode wall andbeing connected to a negative electric voltage with respect to saidcathode to accelerate ions from said perforated cathode wall, saidvessel maintaining the space between said perforated cathode wall andsaid accelerator electrode in the non-breakdown region of the Paschencurve for the particular gas.

8. The device of claim 7 wherein a control grid consisting of aperforated electrode or conducting mesh is placed between saidperforated cathode wall and said negative accelerator electrode tocontrol the current of the extracted ion, the potential of said controlgrid being negative with respect to the cathode and the ion opticaldesign of said control grid being such as to keep ion interception low.

9. The device of claim 7 wherein a control grid consisting of aperforated electrode or conducting mesh is placed between saidperforated cathode wall and said negative accelerator electrode, thepotential of said control grid being substantially equal to cathodepoten tial or being positive with respect to cathode potential. l

1. A hollow cathode plasma discharge device comprising: walls defining ahollow cathode space, said walls comprising an anode wall and a cathodewall, said anode wall and said cathode wall together defining theexterior boundaries of a lowpressure glow discharge plasma, said anodewall being positioned so that it does not substantially intrude into theplasma, the ratio of anode area to cathode area being smaller than unityfor low-pressure glow discharge plasma mode operation; means forinitiating plasma discharge interiorly of said space so that plasmadischarge can be ignited by said initiation means and then transferredto said anode wall; one of said walls being perforated so that particlescan be extracted through said perforation into space exterior of saidperforated wall; an accelerator electrode positioned exteriorly of saidhollow cathode space to accelerate particles passing out through saidwall perforation; and a vessel enclosing at least said perforation andsaid accelerator electrode to maintain pressure within said vessel andwithin said cathode at subatmospheric pressure to cause conditionsbetween said perforation and said accelerator electrode to be outsidethe breakdown region of the Paschen curve for the particular gas.
 2. Ahollow cathode plasma discharge device comprising: walls defining ahollow cathode space, said walls comprising an anode wall and a cathodewall, said anode wall and said cathode wall together defining theexterior boundaries of a low-pressure glow discharge plasma, said anodewall being positioned so that it does not substantially intrude into theplasma, the ratio of anode area to cathode area being smaller than unityfor low-pressure glow discharge plasma mode operation; an auxiliaryanode positioned interiorly of said space for initial ignition of theplasma disCharge so that plasma discharge can be sustained by said anodewall; one of said walls being perforated so that particles can beextracted through said perforation into space exterior of saidperforated wall; an accelerator electrode positioned exteriorly of saidhollow cathode space to accelerate particles passing out through saidwall perforation; and a vessel enclosing at least said perforation andsaid accelerator electrode to maintain pressure within said vessel andwithin said cathode at subatmospheric pressure to cause conditionsbetween said perforation and said accelerator electrode to be outsidethe breakdown region of the Paschen curve for the particular gas.
 3. Thedevice of claim 2 wherein an additional perforated electrode orconducting mesh is placed between said perforated anode and saidaccelerator electrode to serve as a control grid controlling the currentof extracted electrons, the potential of said control grid being betweenthe cathode potential and the potential of the accelerator electrode. 4.The device of claim 3 wherein said perforated wall is a perforated anodewall so that electrons are the particles extracted from the plasma. 5.The device of claim 3 wherein said vessel includes an electrontransmissive window, and said electron transmissive window is saidaccelerator electrode so that electrons accelerated to said window aretransmitted through said window into an atmosphere of arbitrary pressureand composition.
 6. The device of claim 4 wherein said vessel includesan electron transmissive window, and said electron transmissive windowis said accelerator electrode so that electrons accelerated to saidwindow are transmitted through said window into an atmosphere ofarbitrary pressure and composition.
 7. The device of claim 3 whereinsaid perforated wall is a perforated cathode wall so that ions can beextracted from the plasma, said accelerator electrode being spaced fromsaid perforated cathode wall and being connected to a negative electricvoltage with respect to said cathode to accelerate ions from saidperforated cathode wall, said vessel maintaining the space between saidperforated cathode wall and said accelerator electrode in thenon-breakdown region of the Paschen curve for the particular gas.
 8. Thedevice of claim 7 wherein a control grid consisting of a perforatedelectrode or conducting mesh is placed between said perforated cathodewall and said negative accelerator electrode to control the current ofthe extracted ion, the potential of said control grid being negativewith respect to the cathode and the ion optical design of said controlgrid being such as to keep ion interception low.
 9. The device of claim7 wherein a control grid consisting of a perforated electrode orconducting mesh is placed between said perforated cathode wall and saidnegative accelerator electrode, the potential of said control grid beingsubstantially equal to cathode potential or being positive with respectto cathode potential.